1. Field of the Invention
The present invention relates to a switching power supply device that generates and outputs a predetermined voltage by a switching operation, and is capable of a stable control at a time when a load is light.
2. Description of the Related Art
A switching power supply device that controls an output voltage by performing an ON/OFF control for a switching element has been heretofore used for OA equipment, consumer appliances and the like. In recent years, efficiency enhancement of the switching power supply device has been required from viewpoints of considering the environment and saving energy. A control circuit that controls the switching element in the switching power supply device is usually composed of a one-chip integrated circuit, and includes, in an inside of the integrated circuit, a starting circuit for starting the integrated circuit concerned.
FIG. 1 is a circuit diagram showing a configuration of a conventional switching power supply device of a flyback type. As shown in FIG. 1, this switching power supply device includes: an alternating current power supply 1; a bridge rectifier 2; a capacitor 3 for a normal filter; a transformer 4; a switching element 5; a rectifying diode 6; an output capacitor 7, an error amplifier 8; a light emitting diode (LED) 9a and phototransistor 9b of a photocoupler; a capacitor 10; an auxiliary power supply circuit 30; and a control unit 50 for controlling the switching element 5.
Moreover, the switching element 5 and the control unit 50 are provided, for example, in a one-chip semiconductor device. Then, the one-chip semiconductor device includes: as external terminals, an input terminal of the switching element 5 (Drain terminal); an output terminal of the switching element 5 (Source terminal); an input terminal of the auxiliary power supply circuit 30 (Vcc terminal); a feedback signal input terminal (FB terminal); an overcurrent protection terminal (OCP terminal); and a ground terminal of the control unit 50 (GND terminal). Note that the control unit 50 includes: a StartUp terminal connected to the Drain terminal; the Vcc terminal; the FB terminal; and the GND terminal; the OCP terminal; and a DRV terminal for outputting a control signal to the switching element 5.
The transformer 4 has a primary winding P, a secondary winding S1 and an auxiliary winding D, and transmits energy from a primary-side circuit of the switching power supply device to a secondary-side circuit thereof. Moreover, the switching element 5 is connected to the primary winding P of the transformer 4.
The error amplifier 8 is connected between an output voltage terminal Vout and a ground terminal Gnd, and controls a current flowing through the LED 9a of the photocoupler in response to a difference between the output voltage (on Vout) and an internal reference voltage of the error amplifier 8 concerned. A resistor is connected in parallel to the LED 9a of the photocoupler, and the LED 9a gives feedback of an error with respect to the reference voltage of the secondary-side circuit of the switching power supply device to the primary-side circuit thereof. Moreover, the phototransistor 9b of the photocoupler operates in response to light of the LED 9a of the photocoupler. A collector of the phototransistor 9b is connected to the FB terminal of the control unit 50, and an emitter thereof is grounded.
The auxiliary power supply circuit 30 is composed by connecting a diode 11 and a backup capacitor 12 to the auxiliary winding D. Moreover, the auxiliary power supply circuit 30 rectifies and smoothes a voltage induced in the auxiliary winding D of the transformer 4, charges the backup capacitor 12 provided in the auxiliary power supply circuit 30, and supplies power to the Vcc terminal of the control unit 50.
A voltage induced in the secondary winding S1 of the transformer 4 during an OFF period of the switching element 5 is rectified and smoothed by the rectifying diode 6 and the output capacitor 7, and is outputted as the output voltage of the secondary-side circuit from such a Vout terminal to a load.
Moreover, FIG. 2 is a circuit diagram showing an internal configuration of the control unit 50. As shown in FIG. 2, the control unit 50 includes an internal power supply 51, a first inverting circuit 52, a hysteresis comparator 54, a BST comparator 55, a flip-flop 56, a starting circuit 57, a constant current source 60, a transistor 61, an FB comparator 62, an OCP comparator 63, an OR gate 64, an AND gate 65, an oscillator circuit 66, a second inverting circuit 67, first and second drive circuits 68 and 69, and first and second switching elements 70 and 71.
The internal power supply 51 starts the control unit 50 based on power supplied from the Vcc terminal, and supplies, to the entirety of the control unit 50, power necessary for operations thereof. Moreover, the internal power supply 51 detects an output of the hysteresis comparator 54, and operates in the case where the output is a signal of a high (H) level, but stops operating and stops the supply of the power to the entirety of the control unit 50 in the case where the output is a low (L) level.
The hysteresis comparator 54 outputs the signal of the H level in the case where a voltage of the Vcc terminal is, for example, 16.5V or more, and outputs the signal of the L level when the voltage of the Vcc terminal thereafter drops to 10V or less.
The first inverting circuit 52 inverts the output signal of the hysteresis comparator 54 and outputs the inverted signal.
The starting circuit 57 is composed of a constant current source 80 and a switch 81, and flows therethrough a starting current for supplying the power to the internal power supply 51. Here, an input terminal of the constant current source 80 is connected to the StartUp terminal, and receives the supply of the power from the external Drain terminal. In the case where the switch 81 is turned on, the starting circuit 57 supplies the current, which is generated by the constant current source 80, through the Vcc terminal to the backup capacitor 12 of the auxiliary power supply circuit 30, and charges the backup capacitor 12. Moreover, the switch 81 in the starting circuit 57 switches on in the case where the output of the first inverting circuit 52 is the signal of the H level, and switches off in the case where the output of the first inverting circuit 52 is the signal of the L level. Hence, the starting circuit 57 turns on the switch 81 and supplies the starting current to the control unit 50 in the case where the voltage of the Vcc terminal drops to 10V or less and it is necessary to restart the control unit 50.
The constant current source 60 generates a feedback voltage, which comes from the secondary-side circuit, at the FB terminal by the phototransistor 9b of the photocoupler and the capacitor 10, which are connected to the FB terminal on the outside of the control unit 50.
In the transistor 61, a base thereof is connected to the FB terminal. The transistor 61 turns on in response to the feedback voltage of the FB terminal, and an emitter current flows therethrough.
The BST comparator 55 outputs a signal of the H level in the case where a voltage signal corresponding to an amount of the current flowing through the transistor 61 drops to a predetermined voltage value or less. When the load is light (or none), the capacitor 10 is discharged by operations of the LED 9a and the phototransistor 9b, and accordingly, the voltage of the FB terminal drops. Hence, the BST comparator 55 outputs a signal of the L level when the load is usual, and outputs the signal of the H level when the load is light.
The OCP terminal is connected to the Source terminal. A voltage corresponding to an amount of a current flowing through the switching element 5 is applied to the OCP terminal, and the OCP terminal outputs a voltage signal to the FB comparator 62 and the OCP comparator 63.
The FB comparator 62 outputs an H signal in the case where the voltage signal outputted from the OCP terminal exceeds a voltage signal corresponding to the amount of the current flowing through the transistor 61. In such a way, when a voltage value of the voltage signal inputted to the OCP terminal exceeds a voltage value corresponding to a feedback amount from the secondary-side circuit, which is shown on the FB terminal, the FB comparator 62 outputs the signal of the H level to an R terminal of the flip-flop 56 through the OR gate 64, turns off the switching element 5, and constantly controls an output voltage value of the secondary-side circuit.
In the case where the voltage signal inputted to the OCP terminal exceeds the predetermined voltage value, the OCP comparator 63 determines that the amount of the current flowing through the switching element 5 is an overcurrent, and outputs an H signal.
The OR gate 64 outputs an H signal to the R terminal of the flip-flop 56 in the case of having received such an H signal from any one of the BST comparator 55, the FB comparator 62 and the OCP comparator 63.
The oscillator circuit 66 generates a maximum duty cycle signal that decides a maximum duty cycle of the switching element 5, and then outputs the maximum duty cycle signal to the AND gate 65. Moreover, the oscillator circuit 66 generates a clock signal that decides an oscillation frequency of the switching element 5, and then outputs the clock signal to an S terminal of the flip-flop circuit 56. In such a way, the oscillator circuit 66 restricts an ON width of the switching element 5 when the load is excessive, and prevents the overcurrent from flowing therethrough.
The flip-flop 56 outputs a control signal from a Q terminal based on the clock signal inputted to the S terminal and on the signal inputted to the R terminal. An output terminal (Q terminal) of the flip-flop 56 is connected to an input terminal of the AND gate 65. Moreover, an output terminal of the AND gate 65 is connected to the first and second drive circuits 68 and 69 through the second inverting circuit 67. The first drive circuit 68 is connected to a gate terminal of the first switching element 70 made of a P-type MOSFET, and the second drive circuit 69 is connected to a gate terminal of the second switching element 71 made of an N-type MOSFET. The first and second switching elements 70 and 71 are driven alternately in response to an output of the AND gate 65, whereby the switching element 5 is controlled to be turned on/off.
Next, a description will be made of operations of the conventional switching power supply device. First, a sinusoidal voltage outputted by the alternating current power supply 1 is rectified by the bridge rectifier 2, passes through the capacitor 3, and is inputted to the Drain terminal of the switching element 5 through the primary winding P of the transformer 4. Meanwhile, since the switch 81 is turned on, the starting circuit 57 supplies a current to the backup capacitor 12 of the auxiliary power supply circuit 30 by the constant current source 80 and charges the backup capacitor 12 until the voltage of the Vcc terminal exceeds 16.5V. When the voltage of the Vcc terminal exceeds 16.5V, and the internal power supply 51 starts to operate and starts to supply the power to the control unit 50, then the starting circuit 57 turns off the switch 81, and stops supplying the starting current.
When the voltage of the Vcc terminal exceeds 16.5V, and the operations of the control unit 50 are started, then the switching element 5 starts a switching operation. Therefore, the energy starts to be supplied to the respective windings of the transformer 4, and currents flow through the secondary winding S1 and the auxiliary winding D.
The current flowing through the secondary winding S1 is rectified and smoothed by the rectifying diode 6 and the output capacitor 7, and thereby becomes a direct current. This direct current is outputted from the Vout terminal to the external load.
The switching operation of the switching element 5 is repeated, whereby the output voltage of the Vout terminal gradually rises. Then, when the output voltage of the Vout terminal reaches the reference voltage set in the error amplifier 8, the current flowing through the LED 9a of the photocoupler is increased. Then, a current flowing through the phototransistor 9b of the photocoupler is increased. Therefore, the capacitor 10 is discharged, and the voltage of the FB terminal drops. In such a way, the control unit 50 controls the switching element 5 to stabilize the output voltage of the Vout terminal. During a period while the switching operation of the switching element 5 is being stopped, as shown between a time t1 and a time t2 in FIG. 3, the voltage Vfb of the FB terminal rises in such a manner that a current generated by the constant current source 60 charges the capacitor 10.
The current flowing through the auxiliary winding D is rectified and smoothed by the diode 11 and the backup capacitor 12, is fully used as an auxiliary power supply of the control unit 50, and supplies the power to the Vcc terminal. As mentioned above, when the Vcc terminal reaches the starting voltage (16.5V) once, the switch 81 in the starting circuit 57 is turned off. Therefore, the supply of the power to the Vcc terminal after the start of the control unit 50 is performed by the auxiliary power supply circuit 30. A polarity of the auxiliary winding D is the same as that of the secondary winding S1, and accordingly, the voltage of the Vcc terminal becomes proportional to the output voltage of the Vout terminal.
When the load connected to the Vout terminal becomes light, the current flowing through the LED 9a of the photocoupler is increased in response to the error of the Vout voltage with respect to the reference voltage set in the error amplifier 8. Then, the current flowing through the phototransistor 9b of the photocoupler is increased. Therefore, the capacitor 10 is discharged, and the voltage of the FB terminal drops. In such a way, the flip-flop 56 of the control unit 50 is reset, and the control unit 50 stops the ON/OFF control for the switching element 5, or controls the switching element 5 to increase an off-duty time, that is, to perform an intermittent operation.
While the voltage of the FB terminal is dropping and the oscillation of the switching element 5 is being stopped, the current flowing through the LED 9a of the photocoupler is decreased. Then, following such a decrease, the current flowing through the phototransistor 9b of the photocoupler is decreased. In such a way, the capacitor 10 is charged by the constant current source 60, and the voltage of the FB terminal rises. The switching power supply device repeats the above-described operations, and when the load is light, controls the voltage by such an intermittent control to lengthen an OFF time of the switching element.
As shown in FIG. 3, when the switching element 5 is operating, the auxiliary winding D of the transformer 4 charges the backup capacitor 12 and raises the Vcc voltage. However, when the control for the switching element 5 is stopped, the backup capacitor 12 is discharged, and accordingly, the Vcc voltage drops. As mentioned above, when the load is none (or light), the ON/OFF operation of the switching element 5 is stopped, and accordingly, energy charged by the auxiliary winding D is small. Moreover, when such an oscillation stop time of the switching element 5 is lengthened, and the Vcc voltage continues to drop, and reaches Vccoff (10V) as a lowest operation voltage or less, then the internal power supply 51 is stopped. As a result, as shown at a time t4 in FIG. 3, a restarting operation of the control unit 50 becomes necessary. As described above, the fact that the control unit 50 stops the operation thereof and requires the restarting operation every time when the load becomes light during such a standby is disadvantageous for the equipment as the load, which requires a continuous supply of the power, and measures against the above-described fact are necessary.
In Patent Publication 1, a switching power supply is described, which is capable of reducing power consumption in the case where the load side is in a standby state. This switching power supply includes a current stabilizing circuit for the starting current between a positive line and negative line of the primary-side circuit. Then, when the load side is in the standby state, the switching power supply allows a starting constant current to operate intermittently in accordance with the intermittent operation of the switching power supply concerned. Specifically, the switching power supply concerned supplies power to the control circuit for the switching element by using the heretofore present auxiliary power supply circuit when the load is usual, and operates the current stabilizing circuit and supplies the power to the control circuit in the case where the off-duty time of the switching element is lengthened when the load is light. As a result, a voltage equal to or more than the lowest operation voltage required by the control circuit is maintained. In accordance with this switching power supply, a time while the starting constant current is being turned off can be lengthened with respect to a time while the starting constant current is being turned on. Moreover, the starting current is a constant current, whereby the starting current is not increased even if the input voltage of the power supply is raised, and power consumption caused by the starting current can be reduced. As a result, this switching power supply can contribute to energy saving during the standby time.    [Patent Publication 1] Japanese Patent Laid-Open Publication No. 2003-164150